I find it really amazing how a few simple calculations and rules allow us to fairly accurately predict Earth’s surface temperature. We’ll go through these step by step to help review the concepts we talked about in Lecture 9. Remember to ask us for help if you get stuck using the “Ask for help” button at the bottom of your breakout room screen.
Part I – Planetary energy balance
As we saw in lecture, the “solar constant”– the energy received per square meter of the sunlit side of Earth is 1367 W/m2. Averaged over the whole Earth (rather than just the sunlit side), we receive 342 W/m2.
a) The albedo of Earth is about 30% so how much energy is actually absorbed by each square meter of the Earth?
______________W/m2
b) Our planetary energy balance means that:
Energy absorbed from the sun = Energy emitted from Earth out to space
How much energy does the Earth therefore emit? ______________W/m2
c) According to the Stefan-Boltzmann Law what is the temperature of the Earth in Kelvin?
(Tip – if you open excel, you can square root a number to the power 4 by typing the number then “^0.25”)
I = σ T4 where I = radiation in Watts/m2
σ = Stefan-Boltzmann constant = 5.67 x 10-8 Wm-2K-4
T = temperature (Kelvin)
_____________Kelvin
d) What is that temperature in Celsius?
Temperature in Kelvin – 273.15 = Temperature in Celsius
_____________oC
Does this sound reasonable? What are we not accounting for?
Part II – Including the greenhouse effect!
e) As you realized in part d) – we have not accounted for our atmosphere and greenhouse effect. The diagram below shows how we can think about the energy balance if we include the atmosphere.